1. Field of the Invention
The present invention generally relates to a high frequency semiconductor device, and more specifically relates to high frequency semiconductor devices including an HEMT (high electron mobility transistor) that can prevent variations in a characteristic impedance of a high frequency transmission line formed by a wiring on a chip and a ground plate.
2. Description of the Related Art
In the prior art, a face-down mounting structure is known in which a high frequency semiconductor device such as GaAs including an HEMT is laid face-down on a wiring substrate in order to bond a wiring layer on the face of the device to the wiring substrate.
Referring now to FIG. 6, a conventional face-down mounting structure will be explained. FIG. 6 shows a schematic cross-sectional view of a prior art face-down mounting structure. As shown in FIG. 6, a high frequency semiconductor chip 91 has wirings 92 on a front face thereof. A protection layer 93 is provided to cover and protect the wirings 92. An input pad 94 and an output pad 95 are provided at the periphery of the semiconductor chip 91. The high frequency semiconductor chip 91 is laid face-down on a wiring substrate 96 so that the front face having the wirings 92 faces down. The wiring substrate 96 has a ground electrode 97, an input pad 98 and an output pad 99. The input pad 94 and the input pad 98 are aligned with each other, and the output pad 95 and the output pad 99 are aligned with each other, then they are heated and connected by bumps 100 respectively, completing a face-down bonding process.
In this face-down mounting structure, no bonding wire is required and therefore the lead-out of electrodes can be shortened. Further, an active region of the high frequency semiconductor chip that is an exothermic portion is arranged at the side of the wiring substrate 96, and therefore heat release can be easily obtained via the wiring substrate 96.
In this face-down mounting structure for a high frequency semiconductor chip, the ground electrode 97 is provided on the upper face of the wiring substrate or mounting substrate 96 that is opposite the high frequency semiconductor chip 91. This ground electrode 97 and the wirings 92 on the high frequency semiconductor chip 91 together form a high frequency transmission line to realize high-speed transmission of signals on the high frequency semiconductor chip 91.
FIG. 7 shows one example of surface wiring patterns of prior art wiring substrates. A wiring substrate 101 has an input pad 103 at one end, and a plurality of output pads 104 through 106 at another end. An all-over pattern of ground electrode 102 is provided in the middle of the wiring substrate 101. This ground electrode 102 and a wiring (not shown) provided in a high frequency semiconductor chip together form an MSL (micro-strip line) type of high frequency transmission line. Power supply pads 107, 108 for connecting to power supplies are provided at adequate places.
In this face-down mounting structure, however, the sizes of the connecting bumps 100 are non-uniform or the planarity of the wiring substrate is poor, and therefore the distances between the wirings provided on the high frequency semiconductor chip and the wiring substrate, that is the clearances between the wirings and the ground electrode are not constant, resulting in variations in characteristic impedance of the transmission lines.